This invention relates to optical glass for use in an optical apparatus, such as a camera, a telescope, a microscope, a copy machine, a laser device, and the like.
In general, optical glass has a refractive index n.sub.d and a dispersion which are adjusted to a desired optical system in optical apparatus, where the refractive index is determined for the d line of 587.6 nm and the dispersion is defined by an Abbe number .nu..sub.d for the d line. According to this invention, optical glass has a refractive index n.sub.d between 1.8 and 2.2 and an Abbe number .nu..sub.d between 18 and 42.
Heretofore, conventional optical glass or glass composition has been proposed by K. Ishibashi et al in Japanese Patent Publication No. Syo 53-42,330 (42,330/1978) and has been published by M. Faulstich in Glas-Technische Berichte, Volume 34, page 102. Generally, a comparatively great amount of titanium oxide (TiO.sub.2) is added to the proposed optical glass together with lead monoxide (PbO) so as to raise or increase the refractive index. An increase of titanium oxide and lead monoxide undesirably tends to color the optical glass yellow or brown. Such undesirable coloration becomes a bar to optical applications of the optical glass.
In order to avoid undesirable coloration of optical glass with a high refractive index kept unchanged, another optical glass or glass composition is disclosed by I. Masuda et al in Japanese Patent Publication No. Syo 52-28,454 (28,454/1977). The disclosed optical glass comprises, by weight, 60-75% of tellurium dioxide (TeO.sub.2), 5-20% of zinc oxide (ZnO), and 5-20% of a sum of sodium oxide (Na.sub.2 O) and lithium oxide (Li.sub.2 O) and may be called tellurite glass. However, it has been found out that such tellurite glass is too low for optical glass in hardness and encounters difficulty of grinding or polishing. Inasmuch as polishing of optical glass is indispensable for manufacturing an optical lens or the like, the tellurite glass has not been applied to optical use at present.